The 1959 vegetarianism essay. The single most common citation is R. K. Prabhu’s compilation The Moral Basis of Vegetarianism (Navajivan, 1959) — a citation Ralph Keyes had already flagged as unverifiable in The Quote Verifier (2006), p. 74, which is why Wikiquote cross-references him there rather than to endorse the footnote. The essay is freely available and I searched the whole of it: the words “greatness,” “moral progress,” and “animals are treated” do not occur once. The citation is a dead end that has been copied from footnote to footnote for decades.
The Horse Memorial. A commenter on the Animals Matter to God investigation suggested Gandhi may have absorbed the idea from the Boer War Horse Memorial at Port Elizabeth, unveiled in February 1905 while Gandhi was living in South Africa. Its plinth does read “The greatness of a nation consists not so much in the number of its people or the extent of its territory as in the extent and justice of its compassion” ↗. That opening clause is a genuinely striking echo — but no evidence places Gandhi at the monument or in print about it, and the memorial says nothing about how animals are “treated.” Treat this as an unproven suggestion, not a finding.
The wider family of sayings. Wikiquote notes that similar lines circulated through the twentieth century with no Gandhi connection at all, including one credited to the actress Mrs. Fiske in the Salinas Index-Journal of 1 July 1933: “The civilization of any country can be told by the way it treats its animals.” The thought was common property long before it was Gandhi’s.